Skype launched versions of their software for the Mac and Linux.
Check out the Skype release:
Skype for Mac OS X and Linux v1.0
Now Skype is available for Windows, Linux, Mac and even PocketPCs.
Still waiting on a high-powered PocketPC/cellphone hybrid that can handle Skype's processing requirements. I know a model is coming out that should support it, but the model name escapes me at the moment.